Evaluation of Leptospirillum ferrooxidans for Leaching

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The importance of Leptospirillum ferrooxidans for leach processes has been evaluated by studying the lithotrophic flora of three mine biotopes and a heap leaching operation, by percolation experiments with inoculated, sterilized ore, and by morphological, physiological, and genetic investigations of pure and mixed cultures of L. ferrooxidans, Thiobacillus ferrooxidans, and Thiobacillus thiooxidans. In biotopes of 20°C or above, Leptospirillum-like bacteria are as abundant as T. ferrooxidans. Leptospirilli represent at least one-half of the ferrous-iron-oxidizing population. Percolation experiments confirmed this result. Leptospirilli were as numerous as T. ferrooxidans. At reduced temperatures, the generation times of leptospirilli increase more so than those of T. ferrooxidans. At 14°C, Leptospirillum grows slowly and T. ferrooxidans dominates the population. Physiological investigations indicate that L. ferrooxidans is a strict chemolithoautotroph, metabolizing only ferrous iron and pyrite. Even an addition of 0.05% (wt/vol) yeast extract inhibited its growth. The maximum ferrous-iron-oxidizing activity of L. ferrooxidans amounts to about 40% of the activity of T. ferrooxidans. After growth on sulfidic ore, both species exhibit reduced iron-oxidizing activities, L. ferrooxidans exhibiting one-third and T. ferrooxidans exhibiting one-seventh of their maximum activities. Surprisingly, the absolute values are similar. For indirect leaching, L. ferrooxidans is as important as T. ferrooxidans. This was confirmed by the results of percolation experiments. L. ferrooxidans together with T. thiooxidans mobilized metals at least as well as T. ferrooxidans did. The best results were obtained with a mixed culture of all three species.

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